1. Field of the Invention
The present invention relates to a control apparatus arranged to set a target-drive-force-related value corresponding to a target drive force of a vehicle which has a step-variable automatic transmission operatively connected to an engine, and more particularly to a control apparatus operable to control shifting actions of the step-variable automatic transmission.
2. Description of the Related Art
There is known a vehicle wherein an output of an engine is controlled so as to produce a target vehicle drive force which is set on the basis of an operating amount of an accelerator pedal and a running speed of the vehicle, while shifting actions of an automatic transmission are controlled on the basis of an actual value of the accelerator pedal operating amount and/or an actual value of the vehicle speed and according to stored shift-up and shift-down boundary lines (represented by a shift-boundary data map or relationship). JP-2002-161772A discloses an example of such a vehicle. Where the shifting actions of the automatic transmission are controlled on the basis of the accelerator pedal operating amount and the vehicle speed, as in the vehicle of JP-2002-161772A, there is a risk of some drawbacks as described below. For instance, a vehicle has a function of automatically controlling its running state irrespective of the accelerator pedal operating amount, such as a so-called automatic cruising control function of automatically controlling the vehicle running speed, and a so-called VSC (vehicle stability control) function of automatically stabilizing an attitude of the vehicle during turning or cornering of the vehicle. In this instance, there is a possibility that a shifting action of the automatic transmission does not match the vehicle drive force, since it is necessary to set the above-indicated target vehicle drive force irrespective of the accelerator pedal operating amount.
In view of the drawback described above, it is considered to calculate an opening angle of a throttle valve rather than the accelerator pedal operating amount, on the basis of the target vehicle drive force, and control the shifting of the automatic transmission on the basis of the calculated throttle valve opening angle and according to a shift-boundary data map using the throttle valve opening angle. The throttle valve opening angle is a more direct target value representative of the engine output for producing the target vehicle drive force, than the accelerator pedal operating amount.
However, the shift-boundary data map using the throttle valve opening angle suffers from a region in which the target vehicle drive force cannot be produced or realized, due to a torque characteristic of the engine or speed ratios of individual speed positions of the automatic transmission. Therefore, the throttle valve opening angle calculated on the basis of the target vehicle drive force gives rise to a risk of a shift hunting phenomenon depending upon a specific set value of the target vehicle drive force.
Described in greater detail by reference to FIG. 13, a vehicle drive force line F1 at a given vehicle speed V1 in a fist-speed position of the automatic transmission and a vehicle drive force line F2 at the vehicle speed V1 in a second-speed position of the automatic transmission are represented in a two-dimensional coordinate system defined by an axis of the throttle valve opening angle and an axis of the vehicle drive force. A shift-up point U in the coordinate system corresponds a throttle valve opening angle θU at the vehicle speed V1 on a 1-2 shift-up boundary line for a shift-up action of the automatic transmission from the first-speed position to the second-speed position, which 1-2 shift-up boundary line is represented by the shift-boundary data map. When the throttle valve opening angle θ becomes smaller than the throttle valve opening angle θU, a determination to shift up the automatic transmission is made. On the other hand, a shift-down point D in the coordinate system corresponds a throttle valve opening angle θD at the vehicle speed V1 on a 2-1 shift-down boundary line for a shift-down action of the automatic transmission from the second-speed position to the first-speed position, which 2-1 shift-down boundary line is represented by the shift-boundary data map. When the throttle valve opening angle θ becomes larger than the throttle valve opening angle θD, a determination to shift down the automatic transmission is made. The automatic transmission is shifted up to the second-speed position when the throttle valve opening angle θ decreases along the vehicle drive force line F1 and becomes smaller than the shift-up point U, and is shifted down to the first-speed position when the throttle valve opening angle θ increases along the vehicle drive force line F1 and becomes larger than the shift-down point D. Accordingly, there exists a region N indicated by hatching lines, in which no vehicle drive force cannot be produced.
Where a target vehicle drive force A is set within the region N indicated above, a throttle valve opening angle θA calculated on the basis of the target vehicle drive force A in the first-speed position of the automatic transmission is smaller than the shift-up point U, the automatic transmission is shifted up to the second-speed position. On the other hand, and a throttle valve opening angle θD calculated on the basis of the target vehicle drive force A in the second-speed position of the automatic transmission exceeds 100%, that is, is larger than the shift-down point D, the automatic transmission is shifted down to the first-speed position. Where the target vehicle drive force A is set within the region N, therefore, the shift hunting phenomenon takes place.